Electrothermal actuators actuate a shaft in response to electrical energy. The electrical energy causes an electrical heater disposed in a variable-volume chamber filled with a working fluid to heat the working fluid. The working fluid boils and expands, expanding the chamber volume and driving the shaft. The motion of the shaft can be used to drive an external device. Examples of electrothermal actuators are disclosed in U.S. Pat. Nos. 4,029,941, 4,070,859, 4,079,589, 4,104,507, 4,759,189, 4,887,429, and 5,203,171, the disclosures of which are incorporated herein by reference.
Conventional electrothermal actuators drive the shaft only so long as an electrical current is supplied to the heater. In many uses of actuators, it is desired to move the shaft to a prescribed position and maintain the shaft in that position for a lengthy period of time. In these applications, it is necessary to continuously supply current to the heater either to maintain the desired positioning of the shaft or to latch the shaft. Continuous application of electrical energy is disadvantageous because the lifetime of the heater is reduced, the heat generated for long periods may damage parts of the actuator, and the continuous power consumption requires an increased capacity power supply.